Bandpass waveguide filter having iris and posts for resonating fundamental and vanes for absorbing harmonics



pr l 1963 A. D. PETRILLA ET-AL 3, 8

BANDPASS WAVEGUIDE FILTER HAVING ms AND POSTS FOR RESONATING E FUNDAMENTAL AND VANES- FOR ABSORBING HARMONICS Filed Oct. 5, 1959 INVENTOR) AA THO/VV a film/Z114 3,088,082 Patented Apr. 30, 1963 hire BANDPASS WAVEGUIDE FILTER HAVING IRIS AND POSTS FOR RESONA'I'ING FUNDAMENTAL AND VANES FOR ABSORBING HARMONICS Anthony D. Petrilla, Riverton, N.J., and William E. Sentell, Philadelphia, Pa., assignors, by mesne assignments,

to Philco Corporation, Philadelphia, Pin, a corporation of Delaware Filed (let. 5, 1%9, fier- No. 844,262. 12 Claims. (Cl. 333-73) The present invention relates to waveguide filters and more particularly to waveguide filters which exhibit a bandpass characteristic.

The operation of microwave communication systems and the like may he deleteriously aliected by unwanted higher order harmonic components of the signal to be transmitted. Therefore it is desirable that the waveguides which convey the desired signal from one point to another in the system include one or more filters for eliminating these higher order harmonics. In addition these filters should exclude signals at non-harmonically related frequencies which lie outside the desired passband. Microwave filters known in the microwave transmission art suiier from certain disadvantages. For example, one type of filter relies on the fact that higher order harmonios may propagate in higher order modes in a rectangular waveguide which is dimensioned to pass the desired signal. In the description which follows it should be remembered that changing the mode of propagation of a signal within a waveguide does not change the frequency of the signal. It should be remembered also that signals of different frequency may propagate in the same mode or in different modes within a waveguide. In filters of the type just mentioned lossy material is placed in the waveguide parallel to the positions that the maximum electric field vectors of the higher order modes would occupy in the waveguide. This lossy material must be remote \from the position norm-ally occupied by maximum electric field vector of energy in the dominant mode. A typical example of this form of filter comprises a rectangular waveguide in which strips of resistive material are plaged parallel to the narrow walls at the one-quarter and three-quarter points of the broader crosssectional dimens-ion. A filter of this type causes an appreciable loss of energy in the dominant mode. It has the further disadvantage that a special and consequently expensive section of waveguide is required to enclose the lossy dielectri'c material. The section may be several wavelengths long. This is a distinct disadvantage in systems where compactness isdesired.

Other known types of harmonic waveguide filters require waveguide to coaxial transitions and therefore are both complicated and expensive. These types of filters also require considerable space along the waveguide.

Filters employing inductive irises and capacitive posts are usually too narrow band for many communications applications. Furthermore, simple waveguide filters of this type usually will not reject harmonic frequencies.

It is an object of the present invention to provide a simple, compact, and inexpensive bandpass filter element for rectangular waveguide systems.

It is a further object of the present invention to provide a waveguide filter which does not require a special waveguide section for the filter.

An additional object of the present invention is to provide a simple, eflective filter which has a very small longitudinal dimension.

It is a further object of the present invention to provide a waveguide filter having both a relatively wide bandwidth and a relatively sharp cutofi on either side of the selected passband.

It is another object of the present invention to provide a waveguide filter element which may be inserted readily in existing waveguide systems.

Still another object of the present invention is to provide a waveguide filter which greatly attenuates the second harmonic component of the desired signal while passing the desired signal with negligible attenuation.

In general the invention comprises a thin, "conductive diaphragm =or sheet-like member having an aperture formed therein which is resonant at substantially the midpoint of the selected passband. The diaphragm is disposed in a plane perpendicular to the longitudinal axis of the waveguide. Flat strips of lossy material are secured to the diaphragm adjacent the edges of the aperture and parallel to the broad walls of the waveguide. In a pre- :ferred embodiment of the invention the diaphragm is made large enough to he clamped between the flanged ends of two adjacent waveguide sections.

For a better understanding of the present invention together with other and further objects thereof reference should now be had to the following detailed description which is to be read in conjunction with the accompanying drawings in which:

FIG. 1 is an axial view of one preferred form of filter element;

FIG. 2 is an edge view of the rfilter element of FIG. 1;

FIG. 3 is an exploded asymmetric view of the filter element of FIGS. 1 and 2;

FIG. 4 is a view partially in section showing the filter element of FIGS. 1-3 inserted between the flanged ends of two rectangular waveguide sections; and

FIG. 5 is an exploded view of the filter element-waveguide assembly of FIG. 4.

Turning now to FIGS. 1-3, it will be seen that the filter comprises a sheet-like member 14 of conductive material such as brass. stock has been found to be satisfactory for use with waveguide identified by the RETMA or EIA type number WR137 or the JAN type number RG-SO/U. The thickness is not critical but it may affect filter brand'with slightly. For reasons which will appear presently, sheet like member 14 has the shape of a fiat, circular disc. Disc '14 is formed with a centrally disposed H-shaped aperture 18. The broken line 20 in FIG. 1 represents the outline of the inner surfaces of the waveguides normally associated with the filter element of FIGS. 1-3. The dimensions of the waveguide are such that frequencies within the selected passband lie above the cutoff frequency for the dominant mode and below the cutoii frequencies of the higher order modes. It will be seen that the aperture 18 extends across the entire narrow dimension of the waveguide but occupies only approximately the center third of the broader dimension. The portions 25 of the disc 14 which lie between the aperture 1 8 and the narrower walls of the waveguides act as inductive irises. The projections 22 and 24 which separate the upstanding legs of the H-shaped aperture extend from approximately the centers of the broad walls of the waveguide and thus act as capacitive posts. The aperture 18 is dimensioned to be resonant at approximately the midband frequency of the selected passband of the filter. Resonant diaphragms of this generai type are well known in the art and are shown for example in Microwave Duplexers, Smullin and Montgomery, vol. 14, Radiation Laboratory Series, McGraw-Hill Book Company, Inc., 1948, at page 70.

The filter element of the present invention differs from the resonant diaphragm of the prior art in that it includes four dielectric vane members 26-29. Members 26-29 are formed of a microwave absorptive material such as Synthane impregnated with carbon. Synthane is a trade name of the Synthane Corporation for a phenolformaldehyde resin. Vanes 26-29 may be rectangular in shape but preferably they are tapered to minimize any impedance mismatch in the Waveguide. One preferred shape for the vanes is shown in the drawings. As shown, the vanes are straight for approximately one-quarter to one-third their length and then taper symmetrically to a point over the remaining three-quarters to two-thirds of their length. The over-all length of each vane is preferably between a quarter and a half wavelength at midband frequency. Vanes 26-29 are preferably relatively thin, that is, less than an A3 wavelength in thickness.

The vanes 26-29 are cemented or otherwise secured to the conductive disc 14. The outermost edges of vanes 26-29 are preferably positioned so as to be received adjacent the narrow walls of the waveguide with which the filter element is to be associated. The innermost edges of the vanes 26-29 are spaced apart by approximately one-fifth of the broader dimension and hence overlie the aperture 18. The bandwidth of the filter is a function of the amount of projection of vanes 26-29 into the aperture 18. As mentioned above, the vanes are disposed parallel to the broad walls of the waveguide with which the filter element is associated. Preferably the vanes are approximately equidistant from the two broad walls.

The disc 14 may be supported in the waveguide in any convenient manner. In the preferred embodiment of the invention disc 14 is made large enough to be clamped between the flanges of two adjacent waveguide sections. The embodiment shown in FIG. 1 is suitable for use with waveguide identified by I AN type number RG-50/ U and so is formed with circumferentially spaced holes 30 to receive the bolts normally employed to clamp two standard sections of this waveguide together. The size and outline of disc 14 may be varied to suit waveguides of other sizes and with other types of flanges. It also lies within the scope of the invention to secure the inductive irises 25 and capacitive posts 24 to the interior of a, waveguide section individually but in the orientation shown in FIGS. l-3. In this case the lossy vanes 26-29 may be secured either to the irises 25 or to the narrow walls of the waveguide in the positions shown in FIGS. 1-5.

FIGS. 4 and show the filter element of FIGS. l-3 disposed between two sections 40 and 42 of rectangular waveguide. Parts in FIGS. 4 and 5 corresponding to like parts in FIGS. 1-3 have been given the same reference numerals. As shown in FIG. 4 the periphery of disc 14 is clamped between the radially extending flange 44 of waveguide 40 and the similar radially-extending flange 46 of waveguide 42. The holes 30 in disc 14 receive the clamping means 48 which normally secure flange 44 to flange 46. It will be seen from FIG. 4 that the only additional axial space required for the filter element of FIGS. 1-3 is the thickness of the disc 14.

The filter shown in FIGS. 1-5 operates in the following manner: The signal to be filtered is supplied to one of the waveguides 40 and 42. This signal may have frequency components which lie within the selected passband of the filter. It may contain components at frequencies outside the selected passband such as distortion components at harmonics of frequencies within the selected passband. These distortion components may have been introduced by non-linearities in the circuits which prcceded the waveguide.

Each portion 25 of the disc 14 which lies between a narrow wall of the associated waveguide and the aperture 18 acts as an inductive iris. The two projections 22 and 24 extending from the center of the broader walls of the associated waveguide act as capacitive posts. The inductive irises 25 and the capacitive posts 22 and 24 together form a resonant element in the waveguide. This resonant element alone may have a bandwidth of the order of 1% of the center frequency. By adding vanes 26-29 the width of the passband can be increased from. 5% to 10% of the center frequency. The frequency of resonance with vanes 26-29 in place is chosen to be equal to the center frequency of the passband selected for the filter. In general the resonant frequency of the aperture alone will be approximately equal to the resonant frequency with vanes 26-29 in place. The size of the aperture 18 may be so chosen that substantially all of the energy at the resonant frequency incident on the diaphragm 14 is passed through aperture 18. The procedure to be followed in selecting the proper size of a resonant aperture of this type without the vanes 26-29 can be found in the above-cited Microwave Duplexers at pages 71 et seq. Because of the complexity of the mathematics involved, the optimum size and shape of the aperture is best determined empirically.

Energy within the selected passband is restricted by the limited dimensions of the waveguide t0 the TE mode of propagation. Energy in this mode is substantially unaffected by the vanes 26-29 since these vanes lie perpendicular to the electric vectors of the TE mode. Energy at the higher harmonics of the fundamental frequency is free to assume higher order modes of propagation. The detailed effect of vanes 26-29 on signals at frequencies which are integral multiples of frequencies within the selected passband is complex and difiicult to analyze but We have discovered that any energy which may be present at these higher harmonic frequencies is guided and directed by aperture 18 and vanes 26-29 into modes of propagations which are highly attenuated by the vanes 26-29. For example, energy at the second harmonic of the mid-frequency of the selected passband may be directed into a mode at the aperture 18 which resembles the TE configuration. The vanes 26-29 are parallel to and substantially coincident with the maximum electric vector of this mode. Thus energy in a mode resembling the TE mode is highly attenuated by vanes 26-29.

Energy at frequencies outside the selected passband and; not at harmonics of the selected passband is blocked by the high impedance of the resonant aperture 18 at these frequencies.

-An embodiment of the invention in accordance with FIGS. 1 and 3 which was dimensioned for use with RG50 waveguide was found to have a 3 db bandwidth in excess of seven percent of the center frequency. The insertion loss at center frequency was approximately .3 db. The loss at the second harmonic of the midband frequency was approximately 30 db.

While the invention has been described with reference to the preferred embodiments thereof, it will be apparent that various modifications and other embodiments thereof will occur to those skilled in the art within the scope of the invention. Accordingly we desire the scope of our invention to be limited only by the appended claims.

We claim:

1. In a rectangular waveguide, having first and second narrow walls and first and second broad walls, a bandpass filter comprising first and second inductive irises extending from said first and second narrow walls, respectively, first and second capacitive posts extending from approximately the center line of said first and second broad walls, respectively, said inductive irises and said capacitive posts being disposed in the same transverse plane to define an aperture resonant at a selected frequency, a plurality of vanes formed of a material which will absorb microwave energy, said vanes being disposed parallel to and substantially equidistant from said broad walls, said vanes extending to the plane of said aperture and substantially to said narrow walls.

2. In a rectangular waveguide having first and second narrow walls and first and second broad walls, a bandpass filter comprising first and second inductive irises extending from said first and second narrow walls, respectively, first and second capacitive posts extending from approximately the center line of said first and second broad walls, respectively, said inductive irises and said capacitive posts being disposed in the same transverse plane to define an aperture resonant at substantially the mid-frequency of the passband of said filter, a plurality of vanes formed of a material which will absorb microwave energy, said vanes having a width measured parallel to said broad walls which is substantially less than half the width of said broad walls, a length equal to one-quarter to one-half wavelength of energy at the mid-frequency of said selected passband, and a thickness measured parallel to said narrow walls which is small compared to the width of said narrow walls, said vanes being disposed parallel to and substantially equidistant from said broad walls, said vanes extending to the plane of said aperture and substantially to said narrow walls.

3. In a rectangular waveguide having first and second narrow walls and first and second broad walls, a bandpass filter comprising first and second inductive irises extending from said first and second narrow walls, respectively, first and second capacitive posts extending from approximately the center line of said first and second broad walls, respectively, said inductive irises and said capacitive posts being disposed in the same transverse plane to define an aperture resonant at substantially the mid-frequency of the passband of said filter, a pair of vanes abutting each of said inductive irises, said vanes having a width measured parallel to said broad walls which is greater than the width of the associated irises and substantially less than half the width of said broad walls, a length less than a wavelength at said mid-frequency of said passband and a thickness measured parallel to said narrow walls which is small compared to the width of said narrow walls, said vanes being disposed parallel to said broad walls, adjacent said narrow walls and substantially equidistant from said broad walls, the two vanes associated with each iris being disposed on opposite sides of that iris.

4. A filter in accordance with claim 3 wherein each of said vanes has an over-all length of the order of one quarter wavelength and each of said vanes is symmetrically tapered in width over a major portion of its length.

5. A filter in accordance with claim 4 wherein each of said vanes is tapered to a point.

6. A waveguide bandpass filter element adapted to be clamped between two adjacent sections of rectangular waveguide, said filter element comprising a conductive disc, said disc being formed with an H-shaped aperture therein, the dimension of said aperture parallel to the upstanding legs of the aperture being substantially equal to the narrow cross-sectional dimension of the waveguide with which the filter element is to be associated, the overall width of said aperture in a direction perpendicular to said upstanding legs being of the order of one-third the longer cross-sectional dimension of the waveguide with which the filter element is to be associated, said aperture being dimensioned to be resonant at substantially the mid frequency of the passband of said filter element, and a plurality of vanes formed of a material which will absorb microwave energy, said vanes being secured to said disc so as to be supported thereby in a plane perpendicular to said disc, said plane being perpendicular to and substantially bisecting the upstanding legs of said H-shaped aperture.

v7. A waveguide filter element adapted to be clamped between two adjacent sections of rectangular waveguide, said filter element comprising a conductive disc formed with an H-shaped aperture therein, the circumscribing rectangle of said aperture having first parallel sides equal in length to the narrower cross-sectional dimension of the waveguide with which the filter element is to be associated and second parallel sides equal in length to approximately one-third the cross-sectional dimension of said associated waveguide, the projections of said disc which separate the upright legs of said H-shaped aperture extending from substantially the mid-points of said second parallel sides, said aperture being dimensioned to be resonant at substantially the mid-frequency of the selected passband of the filter element, and a plurality of vanes formed of a material which will absorb microwave energy, said vanes being secured to said disc so as to be supported thereby in a plane perpendicular to said disc and perpendicular to and substantially bisecting said first parallel sides of said circumscribing rectangle.

8. A waveguide bandpass filter element adapted to be clamped between two adjacent sections of rectangular waveguide, said filter element comprising a conductive disc formed with an H-shaped aperture therein, the circumscribing rectangle of said aperture having first parallel sides equal in length to the narrower cross-sectional dimension of the waveguide with whch the filter element is to be associated and second parallel sides equal in length to approximately one-third the broader cross-sectional dimension of said associated waveguide, the projections of said disc which impart the H-shaped configuration to said aperture extending from substantially the mid-points of said second parallel sides, said aperture being dimensioned to be resonant at substantially the midfrequency of the passband of said filter element, a pair of spaced vanes secured to one surface of said disc, said vanes being formed of a material which will absorb microwave energy, said vanes being secured to said disc so as to be supported thereby in a plane perpendicular to said disc and perpendicular to and substantially bisecting said first parallel sides of said circumscribing rectangle, the adjacent edges of said pair of vanes being spaced apart by less than the length of said second parallel sides, the more remote edges of said vanes being spaced apart by approximately the broader cross-sectional dimension of the associated waveguide.

9. A waveguide bandpass filter element adapted to be clamped between two adjacent sections of rectangular waveguide, said filter element comprising a conductive sheet-like member formed with an H-shaped aperture therein, the circumscribing rectangle of said aperture having first parallel sides equal in length to a narrower cross-sectional dimension of the Waveguide with which the filter is to be associated and second parallel sides equal in length to approximately one-third the broader cross-sectional dimension of said associated waveguide, the projections of said sheet-like member which impart said H-shaped configuration to said aperture extending from substantially the mid-points of said second parallel sides, said aperture being dimensioned to be resonant at substantially the mid-point of the passband of said filter element, first and second pairs of spaced vanes, each pair of vanes being secured to one surface of said sheet-like member, said vanes being secured to said sheetlike member so as to be supported thereby in a plane perpendicular to said sheet-like member and perpendicular to and substantially bisecting said first parallel sides of said circumscribing rectangle.

10. A filter element in accordance with claim 9 wherein each of said vanes has an over-all length of the order of a quarter of a wavelength and each of said vanes is symmetrically tapered in width over a major portion of its length.

11. A filter element in accordance with claim 10 wherein each of said vanes is tapered to a point.

12. A filter element in accordance with claim 9 where- 2,764,743 in said sheet-like member has the form of a circular disc, 2,943,280 said aperture being disposed substantially at the center of said disc.

References Cited in the file of this patent UNITED STATES PATENTS 2,432,093 FOX Dec. 9, 1947 2,593,234 Wilson Apr. 15, 1952 10 2,684,469 Sensiper July 20, 1954 8 Robertson Sept. 15, 1956 Brill June 28, 1960 FOREIGN PATENTS Great Britain Jan. 23, 1957 OTHER REFERENCES Smullin et al.: Microwave Duplexers, vol. 14, Radiation Laboratory Series, McGraw-Hill Book Company Inc, 1948 (page 70 particularly). 

1. IN A RECTANGULAR WAVEGUIDE, HAVING FIRST AND SECOND NARROW WALLS AND FIRST AND SECOND BROAD WALLS, A BANDPASS FILTER COMPRISING FIRST AND SECOND INDUCTIVE IRISES EXTENDING FROM SAID FIRST AND SECOND NARROW WALLS, RESPECTIVELY, FIRST AND SECOND CAPACITIVE POSTS EXTENDING FROM APPROXIMATELY THE CENTER LINE OF SAID FIRST AND SECOND BROAD WALLS, RESPECTIVELY, SAID INDUCTIVE IRISES AND SAID CAPACITIVE POSTS BEING DISPOSED IN THE SAME TRANSVERSE PLANE TO DEFINE AN APERTURE RESONANT AT A SELECTED FREQUENCY, A PLURALITY OF VANES FORMED OF A MATERIAL WHICH WILL ABSORB MICROWAVE ENERGY, SAID VANES BEING DISPOSED PARALLEL TO AND SUBSTANTIALLY EQUIDISTANT FROM SAID BROAD WALLS, SAID VANES EXTENDING TO THE PLANE OF SAID APERTURE AND SUBSTANTIALLY TO SAID NARROW WALLS. 